This invention concerns systems and apparatus for removal of biological tissue samples and, in particular, systems and apparatus for refractive vision correction.
Various techniques and devices are known in the art for the removal or slicing of thin layers of biological tissue. These instruments are generally referred to as surgical microtomes. When the instruments are specifically designed for the removal of corneal tissue they are often referred to as keratomes or micro-keratomes.
It is often desirable in the course of human therapy to remove a thin layer of biological tissue intact. This is especially true in the course of biopsying tissue specimens for the presence of cancerous or otherwise abnormal cells. For example, in the course of cancer diagnosis and/or treatment, biopsy samples are routinely taken and analyzed for the presence of cancerous cells. The thin tissue biopsies are examined microscopically as a surgical procedure is carried out to ensure that the margins of tumor excision are properly delineated and excessive excisions of healthy tissue are avoided.
Biopsies are also taken for diagnostic purposes in the course of other surgical procedures or in the course of health care, generally. For example, when a gynecological "PAP smear" test turns up a positive result for abnormal cells, cervical biopsy samples are necessary to confirm or negate the presence of cancer cells in the cervix. Biopsies are also taken in the course of various laproscopic examinations for similar purposes in order to confirm or negate the presence of abnormal or cancerous cell conditions.
The examination of biopsy samples is often a tedious process. The ability to take a standardized biopsy sample of a defined volume or thickness of tissue is highly desirous in order to facilitate either manual and automated examination of the biopsy sample. Many conventional microtomes cannot provide this standardization of biopsy samples.
Thus, there exists a need for better surgical resection instruments, generally, which can remove thin sections of biological tissue for examination purposes.
Additionally, in the field of surgical correction of refractive vision disorders, keratomes are used for various purposes. These purposes include the removal of abnormal growths in the cornea, preparation of damaged eyes for corneal transplants, preparation of eyes for other surgical procedures and direct surgical corrections of refractive disorders.
Considerable interest has been recently generated in a variety of techniques for reshaping the cornea for refractive vision correction. These techniques are based on the observation that most of an eye's refractive power is contributed by the corneal curvature itself (with the remaining refractive power being provided by the lens of the eye located inside the ocular globe). For people suffering from near-sightedness (myopia), it has been recognized that a slight flattening of the corneal curvature can correct this condition if properly applied. Conversely, correction of far-sightedness (hyperopia) requires a steepening of the corneal curvature. Correction of astigmatism typically requires more complex reprofiling.
It has been suggested on a number of occasions that it is possible to correct refractive errors by mechanical sculpting of the cornea into an ideal shape and curvature. However, until very recently, there have been no tools suitable for this purpose. The anterior surface of the cornea is covered with a thin layer of epithelial tissue followed by a membrane-like structure known as Bowman's layer. Typically, Bowman's layer is about 30 micrometers thick, although it may vary from as little as 10 micrometers to over 50 micrometers in thickness.
Below Bowman's layer lies the stroma proper of the cornea. This stromal tissue is approximately 450 micrometers in thickness, although it also varies from individual to individual. Stromal tissue is composed of a highly organized matrix of acellular collagen. The Bowman's membrane which lies above it is less regular and denser.
Efforts at mechanical sculpting of the cornea have been largely unsuccessful to date because even the sharpest metal (or even diamond) blades are incapable of producing precise ablations of corneal tissue with the necessary accuracy. The irregularity of Bowman's layer is a further complicating factor which has stymied mechanical attempts at wide-area sculpting of the anterior surface of the cornea.
Attempts have been made to achieve corneal reprofiling in other ways. For example, in a procedure known as radial keratotomy (RK) the cornea is incised with radial cuts which cause the overall structure of the cornea to relax and flatten. While moderate success has been achieved with RK procedures, the result is far from ideal. In such procedures, the anterior surface of the eye contains ruts which resemble the spokes of a wheel after the incisions have healed and the actual corneal curvature is far from the ideal spherical shape that is desired. Nonetheless, millions of people suffering from refractive disorders have undergone RK procedures in the hopes of permanently correcting their vision.
In another approach, designed to avoid the complications of Bowman's layer, a thicker anterior segment of the cornea (typically including Bowman's layer and several hundred micrometers of stromal tissue) is removed, frozen and lathed on its posterior (inside) surface. This reshaped corneal cap or "lenticule" is then thawed and replanted onto the cornea. In this procedure, often referred to as keratomileusis, the integrity of the Bowman's layer is maintained but a much more invasive procedure is required to remove the corneal lamella and then reshape it in a frozen state.
In another alternative surgical procedure, a anterior segment of the cornea is again removed (or partially severed and displaced) so that the stromal bed can be mechanically reshaped (e.g. with a scapel-like instrument). Because Bowman's layer is removed or displaced intact in such procedures, mechanical instruments (microkeratomes and the like) have had moderate success in resculpting the stroma proper. After the stromal bed has been surgically reshaped, the anterior lenticule is replaced. Again, this procedure has the advantage of avoiding mechanical shaving Bowman's layer, albeit at the expense of a deeper penetration into the stroma.
Recently, a new class of tools has become available to ophthalmologists to perform corneal surgery. This class of tools employs high energy pulses of ultraviolet radiation, typically from excimer lasers, to ablate thin layers of corneal tissue by a process known as "photodecomposition." This laser vision correction process relies upon the ability of such laser radiation to remove extremely thin layers of corneal tissue within an exposed area without thermal damage to adjacent tissue. In one type of procedure known as photorefractive keratectomy (PRK), the laser beam is either repeatedly scanned across the cornea or otherwise controlled to expose the cornea to a beam of different shape or size over time so as to effect a cumulative reprofiling of the corneal surface. In many PRK procedures, ablation is largely confined to Bowman's membrane and the laser radiation achieves very smooth and reproducible results. For patients with high dioptric errors, the ablation will also penetrate into the stromal tissue, again with typically very good results. Nonetheless, the systems and apparatus necessary for achieving laser vision correction are extremely complicated to operate and maintain.
In a particular class of PRK procedures known as Laser Assisted In Situ Keratoplasty (LASIK), a keratome is still used to remove (or hingedly displace) an anterior lenticule of the cornea (in much the same way as in the proceddures that involve mechanical scuplting of the stroma) so that the laser can be used to ablate only stromal tissue. Again, like mechanical scuplting procedures, the anterior lenticule is replaced following the procedure with Bowman's membrane intact. This LASIK procedure is also very promising but likewise requires highly complex, difficult to maintain, equipment.
There exists a need for alternative techniques in order to reprofile the cornea. Simple mechanical devices that could achieve the desired degree of accuracy, especially in shaping Bowman's membrane tissue as well as stromal tissue, would satisfy a longfelt need in the ophthalmic community.
Moreover, there exists a need for better keratomes, generally, to facilitate both mechanical and laser vision correction procedures. A better, more accurate keratome, would allow ophthalmic surgeons to perform therapeutic keratectomies (removing small regions of corneal tissue which exhibit abnormal growths or ulcers), resections of anterior corneal segments (as a first step in keratomileusis, stromal sculpting procedures, LASIK procedures and the like) and a variety of other surgical operations on the cornea.